Improve CLI documentation

[klr8]

The CLI documentation should be improved, particularly the fact that you can prepend "+" to task names on the CLI to add them to the default tasks or prepend "^" to task names to remove them from the default tasks.
I had to figure this out by diving into the code and finding https://github.com/pybuilder/pybuilder/blob/master/src/main/python/pybuilder/reactor.py#L424

I'd contribute the relevant docs, but I'm not sure where the documentation is maintained. The stuff in the docs/ folder is WIP apparently and does not corresponding with what is on https://pybuilder.io/

[arcivanov]

Thanks for your comment. I'll update the docs!